I just don't understand why Batman wouldn't be prepared with every gadget he has in his arsenal.

If we really get down to it, Batman could have just shot Bane in the neck with one of those sleeper bat darts before throwing a single punch. But that's not what the scene is about. The scene is about seeing Batman lose in a big way. Being overconfident, thinking the thrashing will be simple, because hey, he beat Ras Al Ghul and Bane won't be as tough as him surely. He was mistaken.